Rare Earth?

Many Christians, including myself, find the arguments expressed in the 2003 book Rare Earth to be a powerful statement of the uniqueness of the Earth in the universe. The thesis of the book, written by two respected University of Washington scientists (Ward, a geologist, and Brownlee, an astronomer) is that the conditions present on the surface of the Earth that make it habitable for advanced life are likely to be very rare, or even unique, in the universe. For a planet to have advanced life–organisms more complex than bacteria–it must orbit at the right distance from the right kind of star, have the right sized moon for stability of orbit, have the right core, and so on. Perhaps, say the authors, we are all alone in the universe after all.

Christians have latched on to many of these same ideas. The writings of Hugh Ross of Reasons to Believe, and of Guillermo Gonzalez (The Privileged Planet, book and DVD) contain many of the same arguments for the uniqueness of planet Earth. The argument goes that apart from the intervention of God, the universe is a dangerous place. Perhaps there is only one place in the universe that is suitable for humans, and that is because God (not chance) has orchestrated it to be so.

I find these arguments to be strong, and Rare Earth is one of my favorite geology books, one I highly recommend. I also acknowledge, both scientifically and theologically, the concept could possibly be flawed. From a theological perspective, we cannot argue persuasively that there is only one Earth-like planet in the universe. Earth might be unique, it might be rare, or the universe might abound with advanced life. Note that I am not talking about intelligent life right now, only advanced, multicellular life. Could it be that our Milky Way Galaxy contains millions of planets that are suitable for everything from bacteria and algae to forests and flocks of birds? Perhaps in the initial creation, and in the future new Heavens and new Earth, the universe was made for humans to explore and thrive in. We just simply do not know.

Not all scientists agree with the rare Earth hypothesis. Many astrobiologists believe that the universe is filled with life. Though I presently find the arguments for a rare, or even unique, Earth to be strong, I do acknowledge that this hypothesis could be wrong.

Part of the problem right now is that we don’t have that much data to work with. We now know of hundreds of stars that have their own solar systems. Since the 1990s, we have been able to detect large planets orbiting around stars by the wobble of the stars produced by the strong gravitational field of the giant planets. Most of these discoveries have been Jupiter-sized planets orbiting their stars at searingly close ranges, and in most of these solar systems there would be no chance for the existence of terrestrial planets. With our current instruments, we cannot planets the size of Earth.

That should change just a little bit in 2009. NASA will be launching the Kepler Mission, which is a space telescope designed to simultaneously observe about 100,000 stars, watching for transits of planets across the faces of these stars. As even an Earth-sized planet passes directly between the star and the Earth, there will be a slight diminishing of the intensity of light observed. The Kepler Mission will not allow us to see the planet directly, but will enable us to determine the presence of the planet, and to infer its size and orbit. Knowing the nature of the star itself, and the parameters of the planet’s orbit, we would be able to determine if the planet were in the star’s “habitable zone,” that not-too-close, not-to-far region that allows liquid water to exist on a planet’s surface.

This won’t tell us whether the planet has life; spectrometers sensitive enough to detect things like an oxygen-rich atmosphere at distances of many light years lie in the future. What it will enable is a tightening of some of the variables that go into the debate between a rare Earth and a green universe.

As Christians, we can rejoice in God’s creation whether we see God’s providence in an Earth that is a unique, protected oasis in a hostile universe, or if we discover a multitude of worlds touched by God’s creative Spirit (but still oases in a hostile universe). The rare Earth hypothesis may still turn out to be sound, but I’m not going to have any kind of theological struggle if it turns out to be wrong.